( 895 ) 
The constant b is proportional to the density of the medium, to 
the thickness of the layer, and to the average mass per scattering 
partiele. The greater each of these quantities is, the smaller will be 
the intensity of the emergent light (for any value of 4). The only 
quantity strongly variable with 2 in the small spectral region consi- 
dered, is the factor FR’, in case there is an absorption line. 
n—l 
The upper part of fig. 4 represents A= Kasa function of 2°). 
The origin of co-ordinates corresponds to the wave-length 2, of a 
free vibration; the line P, ?,, having the approximately constant 
; nl © 
ordinate a would be the dispersion curve if there were no 
absorption line at A, 
Supposing ¢ to be sufficiently great, and 
R not too small, we may, in the deno- 
minator of (16), neglect. @ in comparison 
with 4/2; so R, is about inversely pro- 
portional to A. The light is therefore 
more weakened by scattering on the red 
side than on the violet side of the absorp- 
tion lines, if (as supposed in the figure) 
we have n, >1. With very strong lines 
this difference will, however, partly be 
neutralized, because, according to § 3, 
the minimum of the refractive index sinks 
| 
| 
| 
further below 7, than the maximum rises 
above 7,- 
Taking this into consideration, and 
preliminarily fixing our attention on the 
Fig. 4. effect of scattering only, leaving that of 
<< 
K-- ------W) 
absorption aside, we may represent the intensity of the emergent 
light by a curve of the shape d, d, d, d, d,; (fig. 4, lower part). 
The top d, (corresponding to / = 0) does not coincide with 2,, but 
is a little displaced toward the violet. If, therefore, one would imagine 
the region between d, and d,, where the loss of light due to scatter- 
ing passes through a minimum, to be an “emission line’, one would 
have to assign to it a smaller wave-length than to the absorption 
1) In this figure A increases from the left toward the right; the succession of 
the kinds of light is therefore opposite to that in the figures 1 and 2, where the 
frequencies » were chosen as abscissae. 
